Optical compensators are used to improve the optical properties of liquid crystal displays (LCD), such as the contrast ratio and the grey scale representation at large viewing angles. For example in uncompensated displays of the TN or STN type at large viewing angles often a change of the grey levels and even grey scale inversion, as well as a loss of contrast and undesired changes of the colour gamut are observed.
An overview of the LCD technology and the principles and methods of optical compensation of LCDs is given in U.S. Pat. No. 5,619,352, the entire disclosure of which is incorporated into this application by way of reference.
As described in U.S. Pat. No. 5,619,352, to improve the contrast of a display at wide viewing angles a negatively birefringent C-plate compensator can be used, however, such a compensator does not improve the greyscale representation of the display. On the other hand, to suppress or even eliminate grey scale inversion and improve the grey scale stability U.S. Pat. No. 5,619,352 suggests to use a birefringent O-plate compensator. An O-plate compensator as described in U.S. Pat. No. 5,619,352 includes an O-plate, and may additionally include one or more A-plates and/or negative C-plates.
The terms ‘O-plate’, ‘A-plate’ and ‘C-plate’ as used in U.S. Pat. No. 5,619,352 and throughout this invention have the following meanings. An ‘O-plate’ is an optical retarder utilizing a layer of a positively birefringent (e.g. liquid crystal) material with its principal optical axis oriented at an oblique angle with respect to the plane of the layer. An ‘A-plate’ is an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer, and its ordinary axis (also called ‘a-axis’) oriented perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light. A ‘C-plate’ is an optical retarder utilizing a layer of a uniaxially birefringent material with its extraordinary axis (also called ‘c-axis’) perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light.
As an O-plate retarder for example an optical retardation film (hereinafter abbreviated as ORF) comprising a layer of a liquid crystal or mesogenic material with tilted or splayed structure can be used.
As an A-plate retarder for example a uniaxially stretched polymer film, like for example a stretched polyvinylalcohol (PVA) or polycarbonate (PC) film, can be used. Alternatively, an A-plate retarder may comprise for example a layer of a positively birefringent liquid crystal or mesogenic material with planar orientation.
As a negatively birefringent C-plate retarder for example a uniaxially compressed polymer film can be used. Alternatively, a negatively birefringent C-plate may comprise for example a layer of a liquid crystal or mesogenic material with a planar orientation and a negative birefringence. Typical examples of negatively birefringent liquid crystal materials are various kinds of discotic liquid crystal compounds.
In addition to U.S. Pat. No. 5,619,352, optical compensators comprising one or more O plates are described in prior art in WO 97/44409, WO 97/44702, WO 97/44703 and WO 98/12584, the entire disclosure of which is incorporated into this application by way of reference. WO 97/44703 and WO 98/12584 further suggest to use tilted or splayed O plates in combination with a planar A plate.
WO 97/44703 reports that the use of a compensator comprising an O plate in combination with a planar A plate, wherein the principal optical axes of both ORFs are oriented at right angles to each other, allows particularly good compensation of a TN-LCD, as it simultaneously reduces the angle dependence of the contrast and the grey scale inversion in the display.
However, when using compensators as described in the above mentioned prior art in combination with liquid crystal displays, especially TN or STN-displays, the improvements of the optical properties of the display, like contrast at wide viewing angles, grey scale level stability, and suppression of grey scale inversion, are still far from sufficient for most applications.
Therefore, it is desirable to have available improved optical compensators to further improve the optical performance of LCDs.
Definition of Terms
In connection with optical polarization, compensation and retardation layers, films or plates as described in the present application, the following definitions of terms as used throughout this application are given.
For the sake of simplicity, the term ‘liquid crystal material’ is used hereinafter for both liquid crystal materials and mesogenic materials, and the term ‘mesogen’ is used for the mesogenic groups of the material.
The terms ‘tilted structure’ or ‘tilted orientation’ means that the optical axis of the film is tilted at an angle θ between 0 and 90 degrees relative to the film plane.
The term ‘splayed structure’ or ‘splayed orientation’ means a tilted orientation as defined above, wherein the tilt angle additionally varies monotonuously in the range from 0 to 90°, preferably from a minimum to a maximum value, in a direction perpendicular to the film plane.
The term ‘low tilt structure’ or ‘low tilt orientation’ means that the optical axis of the film is slightly tilted or splayed as described above, with the average tilt angle throughout the film being between 1 and 10°.
The term ‘planar structure’ or ‘planar orientation’ means that the optical axis of the film is substantially parallel to the film plane. This definition also includes films wherein the optical axis is slightly tilted relative to the film plane, with an average tilt angle throughout the film of up to 1°, and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel, i.e. with zero tilt, to the film plane.
The average tilt angle θave is defined as follows
      θ    ave    =                    ∑                              d            ′                    =          0                d            ⁢                        θ          ′                ⁡                  (                      d            ′                    )                      d  wherein θ′(d′) is the local tilt angle at the thickness d′ within the film, and d is the total thickness of the film.
The tilt angle of a splayed film hereinafter is given as the average tilt angle θave, unless stated otherwise.
The term ‘helically twisted structure’ relates to a film comprising one or more layers of liquid crystal material wherein the mesogens are oriented with their main molecular axis in a preferred direction within molecular sublayers, with this preferred orientation direction in different sublayers being twisted around a helix axis that is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes orientations where the helix axis is tilted at an angle of up to 2° relative to the film normal.
The term ‘homeotropic structure’ or ‘homeotropic orientation’ means that the optical axis of the film is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes films wherein the optical axis is slightly tilted at an angle of up to 2° relative to the film normal, and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel, i.e. with no tilt, to the film normal.
For sake of simplicity, an optical film with a tilted, splayed, low tilted, planar, twisted and homeotropic orientation or structure is hereinafter being shortly referred to as ‘tilted film’, ‘splayed film’, ‘low tilt film’, ‘planar film’, ‘twisted film’ and ‘homeotropic film’, respectively.
Throughout this invention, both a tilted and a splayed film will also be referred to as ‘O plate’. A planar film will also be referred to as ‘A plate’ or ‘planar A plate’. A low tilt film will also be referred to as ‘low tilt A plate’. A twisted film will also be referred to as ‘twisted A plate’.
In tilted, planar and homeotropic optical films comprising uniaxially positive birefringent liquid crystal material with uniform orientation, the optical axis of the film as referred to throughout this invention is given by the orientation direction of the main molecular axes of the mesogens of the liquid crystal material.
In a splayed film comprising uniaxially positive birefringent liquid crystal material with uniform orientation, the optical axis of the film as referred to throughout this invention is given by the projection of the orientation direction of the main molecular axes of the mesogens onto the surface of the film.
The term ‘film’ as used in this application includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.
The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal or mesogenic compound’ should denote materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal phase behaviour. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized.